White matter tract ( Swanson & Bota, 2010 ) : A recognizable division of white matter (Meckel, 1817) in the nervous system (Monro, 1783) that can be bordered by gray matter (Meckel, 1817), another white matter tract(s), or non-neural tissue. Borders between white matter tracts are determined by defined structural landmarks and are commonly arbitrary. White matter tracts may be homogenous or heterogeneous. A homogeneous tract only has one specific mesoconnection (Thompson & Swanson, 2010) within it, whereas a heterogeneous tract has two or more specific mesoconnections within it. The traditional way to view vertebrate white matter tracts is with a myelin stain, supplemented with a reduced silver stain (Brodal, 1981, p. 5). There are many general terms for a white matter tract; some examples include pathway, bundle, fascicle, funiculus, column, peduncle, decussation, commissure, and nerve, whereas others have specific names like corpus callosum, fornix, and internal capsule. As a complete set, white matter tracts can be arranged in various ways, for example, strictly topographically into transverse tracts, longitudinal tracts, and local tracts. White matter tracts were distinguished in the second century by Galen (c177; see translation by Singer, 1999, pp. 231, 234) for macrodissected nonhuman mammals, and were referred to as tracts as long ago as Willis (1664; see translation by Pordage, 1681, pp. 61, 91) for macrodissected adult humans and other large mammals.

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